Built some DIY Bass traps. Measurements are now worse.

Bass Traps should be standing up and running floor to ceiling in the corners. But you really need to experiment with placement and you need more than just one or two 2x4ft panels. You need a lot of them in the right places to make a difference, you can't just put them were ever and expect and improvement.

Best ever paper I have read on Bass Traps:

http://accucalhd.com/documents/audio/Acoustical%20Concepts%20For%20Home%20Theater.pdf

Very good reading. For good results, you need a lot of absorption in the right places. 3 traps is a drop in the bucket in a room that size. gtp's Superchunk traps pictured above are a terrific step in the right direction.

JSS

Well. Atleast that 120hz peak is damped abit and everything above it looks better. But it has also gotten abit worse between 80 and 110hz (ish) and below 80 you probably dont have bass traps that can affect that much.

Yes, measurements now aren't totally meaningful without subs in place.

Also, your waterfalls have too much frequency resolution. You can have time resolution or frequency resolution, but you can't have both. The good news is that in the bass, frequency response gives you a really good idea of time decay.

Why's that?

The best orientation is whatever works best with acceptable aesthetics.

The problem with the traps is that they are not placed in the right positions. You want to place them floor to ceiling, as someone else mentioned, and preferably in the corners. Having them lay flat on the floor and in the middle of the wall is not what I would call a good position. You should re-arrange them to go in the corners, floor to ceiling. Then, once you cover floor to ceiling in the 4 corners, (note that you might have to stack two of your 4' x 2' traps on top of each other in each corner in order to reach floor to ceiling) you can them start by placing them along with mid-way point of the side walls, then along the back wall. Hope this helps!

Sorry room acoustic treatments and aesthetics DO NOT go together. You can't have both looks and good treatment (although I for one think a well treated room with nicely done treatments looks nice). For a persons goal that is looks above treatments then yes the way you described it that would be the best way. If you want the best acoustic treatment then your going to have to give up the nice aesthetics. Bass "gathers" in corners so you need to work on filling in the corners top to bottom (fronts most important) with either superchunk style (17x17x24" triangles stacked) rolled insulation stacked or thick panels with an air gap behind them.

Having thick 6" panels on the back wall can help with decay times which can possible tame some boomyness and give the user the tight bass response in the midbass.

With room treatments either do it right or don't bother. Throwing them randomly around the room for aesthetics is just a waste of time.

Traps are only as effective as their placement. Find out how to place them here:
http://www.realtraps.com/articles.htm

From everything I've read, bass likes to collect in corners. You may notice that if you stand in a corner of your room you will notice a lot more bass. This is why traps work most effectively in corners. It's where the bass likes to hang out.

For the sake of keeping it simple, this is great advice. Bass congregates in corners as room modes terminate in corners. Placing absorbers in the corner aid in dampening modes from multiple boundaries. IOW - its a good place for bass trapping.

This is not correct. Frequency response is the averaging of the first (however many you choose) milliseconds and ignore the rest of the decay. Frequency response does NOT show you decay time - waterfall graphs do.

Bingo! OP - he speaks the truth! But I'll comment further:

If you're minimally treating the space as you've outlined, traps that far back likely won't make huge differences. You should use the panels you have in the front of the actual room itself. You will likely hear a bigger difference with better placement. I'd suggest placing them at your first reflection points for your mains, and in the front corners wherever possible. See the following video to find your first reflection points: http://gikacoustics.com/video-early-first-reflection-points/

A couple of points:
Measurements would be more beneficial with a sub if you plan on having a sub in the room
Audyssey should be turned off when doing these measurements as you've done (you could do Audyssey on vs off measurements as well)
Smoothing doesn't apply to your waterfall graphs. Smoothing only affects Frequency Response graphs in REW.
You're viewing angle on these graphs allows for hardly ANY useful information! This would be a much better view:


Looking at the graph this way, we can actually see your room resonances and the real time scale they occupy (a lot!)

Yes, these will improve your high end response. If you make them thick you might get some reduction at the lower frequencies. (Sorry I didn't read this part when I referenced early reflection points above, but i'll keep that in the post for others)

Your null that increases in depth is just likely either a testing anomaly or you moved the microphone between tests (The mic needs to stay in the exact same spot!). Your signal to noise ratio is also not great in the tests either, make sure you're running your speakers LOUD and you can further improve the S-to-N ratio by using more sweeps per measurement (8 is better than 4 is better than 2). For low end, you can test with both speakers on (it will drive your modes harder so you have a better view of the low end).

the kind of bass traps that you are using rely on air moving.

as you get close to a boundary, there is very little air movement, so placing said traps on a boundary will do little if anything.

air is moving with greatest velocity at 1/4 wavelength from a boundary. try placing your trap in the air and that distance from the boundary.

the only way that i know of to absorb bass at a boundary is to make it very limp so that the boundary itself will actually flex, absorbing the sound energy and turning it into heat.

jonathan, a couple things....

First off, nice job of starting with a measured problem. Granted, you will need a very good subwoofer to show all the sonic issues, but there is one thing you've learned:
Your DIY "bass traps" don't trap much bass. That doesn't mean you made them wrong as placement is also critical.

Your data shows a drop in output in the 75-150Hz range, but nothing below 75Hz, as expected from a 2" absorber mounted on a surface. You have DIY absorption panels, they do work, but how they work depends on where you put them.
- these are resistive panels, so they work best where air velocity is highest.
- boundaries don't move, so air velocity will always be low against a wall or in a corner. These are pressure maxima.
- to get a resistive panel to work on a surface, space it away from the sorface.
- Unless you get very thick, and spaced quite far away, low frequency absorption will be limited.

Not only will it not trap bass, it will trap lots of higher frequencies, leaving you with a dead room that's boomy.

In addition, without a sub, your waterfalls are only showing harmonics. If I take the room at 30x11x8 (guess for ceiling):
- your peak in the low 40's is (200) (2nd harmonic, long direction)
- your peak at 55Hz is a combination of (300) and (010)
- your mid-70Hz peak is (400) and (001)
- and the 100Hz peak you actually pushed down is (500) and (600) with some (020) thrown in, plus contributions from the stub wall.

Note that I didn't mention (100), your room fundamental mode at 18Hz as you haven't excited it yet. Get a room mode calculator and try it for yourself.

If you want to lower sound levels overall, broad band absorption panesl are great. If you want to absorb upper bass, panels can be configured (thickness, spacing from walls) so they are very effective, but they will not go deep enough to "trap" low bass without greatly deadening the room.

Panels can be modified, or more accurately, their parts used to make effective bass traps.

The corner placement is not bad, but if you cover the face with a skin of masonite, and seal the air space in the corner behind it, you get a real bass trap, a diaphragmatic trap that absorbs bass but not upper frequncies. Perforating this board allows you to tune beyond a basic mass density dependence, but now you're getting into types of sharply tuned absorbers.

Lots of options for real bass energy supression, but you have to look. Your data is a great first step! Here's a link that'll tell you lots more about these devices, starting about page 185,
http://andrealbino.wikispaces.com/file/view/Master+Handbook+of+Acoustics+-+5th+Edition+-+F.+Alton+Everest,+Ken+C.+Pohlmann.pdf

Have fun,
Frank

Ok, so I typed loosely. Not exactly incorrect. What I meant was, given a linear system, which is a decent representation of bass response in a room, frequency response gives a good idea of time response. Peaks in frequency response show ringing in temporal response. To sum up, given a linear system, if you have either the frequency or time response, you can infer the other. Is this really debatable?

Also, waterfalls with high frequency resolution lose time resolution. They still look pretty, but do not show ringing properly. Good waterfalls (meaning good time resolution) have crappy, over-smoothed-looking frequency responses at time zero.

EDIT: By the way, below is what the author of REW has to say about waterfalls vs. decay time...

Waterfall plots cannot be used to read decay times, unfortunately. The plot is produced by moving a window through the response and plotting the frequency content within that window. Using longer windows makes decays appear to take longer, which is at its heart a manifestation of the time/frequency trade-off of these kinds of analysis. Longer windows give increased frequency resolution, but correspondingly reduced time resolution.
Edited by tlag - 2/25/13 at 12:25pm

Bass congregating in corners is sorta true, but using that to justify resistive damping is sorta wrong. Bass pressure occurs in corners, but that's zero velocity. The two are 90deg apart, so one's peak is the other's minimum. As it turns out, resistive trapping in corners still works, but only because the traps have non-zero depth, meaning they pick off some of the velocity component. Ideally, one would use pressure-type absorbers in corners.

EDIT: by "resistive" above, I mean flow-resistive, of course...
Edited by tlag - 2/26/13 at 9:40am

Keep in mind, 6" is one wavelength at 2.2 KHz.

Not exactly gonna trap any "bass" with something that is such a small fraction of a low frequency's wavelength.

Even the thickest traps don't trap bass, they trap midbass. Since midbass is what causes boomy response that's not a problem. But you don't need for the trap to be a significant portion of a wavelength thick either. If you did even midbass traps wouldn't work.

These are easy to make and get the job done
http://www.avsforum.com/t/255432/acoustical-treatments-master-thread/9150#post_22555253

tlag,

Didn't mean to step on toes. I'm not as familiar with the members on this subforum as I need to be it seems. With "loose typing" (I like that term...might use it myself) such a short post, I mistook it for heresay instead of an original comment.

6" of Safe'n'Sound will work to a much lower frequency than 2.2kHz!

http://www.stanleyhallstudios.co.uk/pacalc/pacalc.php?e=h&r=r&m=4&l1=6%20inches%20Safe%20n%20Sound&s11=2&d11=152.4&v11=8000

Though I think you're stating that you aren't going to get much action sub-100 Hz, which I agree. But 100 Hz -> 200 Hz Safe'N'Sound should do 'alright' and above 200 Hz should work great!

I agree with the end of your post, but will disagree with the first portion a bit. Thick velocity based porous absorption in the corners can still give useful absorption down to 50 Hz. Not sure I'd quality <100 Hz as "Mid-bass"

Great thread we got going here, guys! Lots of good information, debate, and knowledge to be had in this thread!

mtbdudex,

I noticed that your frequency response is rising considerably going from 20Hz to 10Hz in your non-EQed graphs. Is that unusual to see?

That depends on the thickness of the material used, and the absorption coefficient. There are charts out there you can use to calculate what a particular trap material and thickness is capable of. I'm not seeing that. The yellow and purple traces should be virtually identical in the lowermost frequencies, so close that they would overlay and appear as one. What I see tells me that the mic positioning and/or drive level wasn't identical for the two measurements, leading to erroneous conclusions. A deviation of even a few inches in the mic position can make a big difference, as Ethan Winer explains:
http://www.ethanwiner.com/believe.html

Also when analyzing decay times, modal ringing, waterfalls etc, a higher spl measurement is more useful. Shoot for something close to 90db assuming a 40-50db ambient floor.

For anyone interested in what a good (and bad) waterfall plot looks like, I highly recommend studying Figure 13.23 here (from Toole's "Sound Reproduction"). The description is quite helpful as well.

Basically, you want to see some undulations vs. time, and not just smooth vertical ridges that seem to decay monotonically vs. time.

Up against the boundary yes.
However, you place that same 6" of insulation ... well away from the boundary and voilà, it becomes a tremendous effective absorber of the problematic modal range of typical residential rooms. A bass trap.


Some good advice given above, but lets examine the most important aspects of bass trapping with porous absorbers;
Ideally, place the porous material where it works the best .. at the 1/4 wave point off the boundary ... where the velocity is greatest.

If you chose to absorb, do so effectively. Poorly executed porous absorbtion of sidewalls filters the reflected energy in a manner whereby the resultant sound will be lifeless, muddy, bass heavy mess. Even sidewall panels need as much extension into the LF as possible. Oftentimes, prudent use of coverings can elicit a spectrally balanced effectiveness, however this is best confirmed with measurments.

Porous absorption can be effective quite deep, I've seen measured results down into the 30hz range. The thicker the better, if you go very thick, you use the less dense material. If you opt for less thickness, utilizing an air gap multiplies the effectiveness greatly.

As a rule of thumb, based on how much space off the surface you can use;
up to 4", use 3 lb fiberglass equivalent (4lb) mineral wool
4-6", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap
6"-12", use 4" 3 lb fiberglass or 4 lb mineral wool with a gap off the surface, or fill the space with loose/fluffy
12" and more, fill the space with pink fluffy

For corners;
One can either straddle a corner with a 4"-6" thick panel, or employ stacked fluffy, stacked rigid 703, superchunk style.
But be mindful that filling the corner is better than using a 4"-6" thick panel, however only somewhat better. Instead of filling the entire area, one may be better off utilizing the same material elsewhere, covering more corners.

Best case, fill all the corners as deep as you can spare. Remember, the thicker the porous absorber, the less dense (gas flow resistivity) the material is optimum. For example, if one could spare 18" across the entire back wall of a studio, pink fluffy will out perform 703 rigid fiberglass.


Acoustics is anything but intuitive. And from everything I've learned, the lowest range of frequency effectiveness extends much lower than typical 1/4 wave calculations would suggest. When including the effects of the porous material's speed of sound, the low end effectiveness becomes significantly better, and lower in frequency then previously understood. It's simple; the material is acoustically thicker than it's physical dimensions would suggest.

The 1/4 wave distance is shorter in pourous absorption, because the speed of sound in porous absorbent material is slower than it otherwise would be in air. Slowing the speed down, shortens the wavelength, thus shortening gap needed off the wall/boundary needed to affect any one frequency.
I've read individuals studying such things suggesting the speed/wavelength shortening can be as much a 1/3, correlating with a measured results showing that if the thickness of an effective porous absorber is at least 7% of the wavelength, then one could achive significant absorption approaching 100%.

I don't know for sure, but I seen measured results that show effectiveness much deeper than one would intuitively think would be possible ... using the typical 1/4 wave effectiveness mindset. So what I've since read about the speed of sound within the material, certainly is interesting and makes sense.


Largely, in the typical small rooms that comprise our HTs and listening rooms, one can hardly have too much bass trapping. However, enormous benefits are had with when one simply achieves an adequate level of bass damping/trapping. And the cool thing is, the cheapest stuff (pink fluffy) is the most effective. Any large surfaces, ceiling included need treatment. One needn't be concerned about over doing it either, as the traps can be faced with diffusion, plastic, wood slats, anything that's invisible to the deep stuff. That way any amount of energy in the MF/HF can be retained, if one desires.


Thanks


Here's a nice example.